Chemical Abundances in the Universe: Connecting First Stars to Planets Proceedings IAU Symposium No. 265, 2009 K. Cunha, M. Spite & B. Barbuy, eds. c  International Astronomical Union 2010 doi:10.1017/S1743921310000633 Extremely metal-poor stars in dwarf galaxies Anna Frebel 1 , Joshua D. Simon 2 , Evan Kirby 3 , Marla Geha 4 , and Beth Willman 5 1 Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA email: afrebel@cfa.harvard.edu 2 Observatories of the Carnegie Institution of Washington, Pasadena, CA 91101, USA email: jsimon@ociw.edu 3 California Institute of Technology, Pasadena, CA 91106, USA email enk@astro.caltech.edu 4 Astronomy Department, Yale University, New Haven, CT 06520, USA email marla.geha@yale.edu 5 Haverford College, Haverford, PA 19041, USA email: bwillman@haverford.edu Abstract. We present Keck/HIRES spectra of six metal-poor stars in two of the ultra-faint dwarf galaxies orbiting the Milky Way, Ursa MajorII and Coma Berenices, and a Magel- lan/MIKE spectrum of a star in the classical dwarf spheroidal galaxy (dSph) Sculptor. Our data include the first high-resolution spectroscopic observations of extremely metal-poor stars ([Fe/H] < -3.0) not belonging to the Milky Way (MW) stellar halo field population. We obtain abundance measurements and upper limits for up to 26 elements between carbon and europium. The stars span a range of -3.8 < [Fe/H] < -2.3, with the ultra-faints having large spreads in Fe. A comparison with MW halo stars of similar metallicity reveals substantial agreement between the abundance patterns of the ultra-faint dwarf galaxies and Sculptor and the MW halo for the light, α and iron-peak elements (C to Zn). This agreement contrasts with the results of earlier studies of more metal-rich stars (-2.5  [Fe/H]  -1.0) in more luminous dwarfs, which found significant abundance discrepancies with respect to the MW halo data. The abundances of neutron-capture elements (Sr to Eu) in all three galaxies are extremely low, consistent with the most metal-poor halo stars, but not with the typical halo abundance pattern at [Fe/H]  -3.0. Our results are broadly consistent with a galaxy formation model which predicts that massive dwarf galaxies are the source of the metal-rich component ([Fe/H] > -2.5) of the MW inner halo, but we propose that dwarf galaxies similar to the dSphs are the primary contributors to the metal-poor end of the metallicity distribution of the MW outer halo. Keywords. stars: abundances, stars: Population II, Galaxy: formation, Galaxy: halo, galaxies: dwarf, (galaxies:) Local Group, galaxies: stellar content, (cosmology:) early universe 1. Introduction According to prevailing theories of the early Universe, the recombination of the plasma of electrons and light nuclei (mostly H and He, with trace amounts of Li) emerging from the Big Bang was followed by the Dark Ages, during which space was filled with gas and radiation but no sources of light. This era ended with the formation and ignition of the very first (Pop III) stars. These presumably very massive (∼ 100 M ⊙ ; Bromm & Larson 2004) objects soon died as energetic supernovae. The freshly synthesized metals expelled by those explosions provided the first effective coolants for the interstellar medium. This pollution set the stage for the second generation of stars (Pop II), which included stars over a wide range of masses. The low-mass (M  0.8 M ⊙ ) early Pop II stars, which 237 https://www.cambridge.org/core/terms. https://doi.org/10.1017/S1743921310000633 Downloaded from https://www.cambridge.org/core. IP address: 3.235.21.12, on 22 May 2020 at 20:45:48, subject to the Cambridge Core terms of use, available at